We propose to evaluate two-photon microscopy of BPD-MA to study subcellular localization under a variety of conditions. Benzoporphyrin Derivative is a second-generation photosensitizing compound currently undergoing phase II clinical trials for use in treating psoriasis, age-related macular degeneration as well as cutaneous oncology. Previous research has demonstrated that this drug is a potent sensitizer, with efficacy in cell killing demonstrated at concentrations below 200 ng/ml. Its primary subcellular target is believed to the mitochondria, where it has been demonstrated that BPD-MA can photodynamically uncouple oxidative phosphorylation in the isolated rat mitochondria. Our initial experiments have sought to optimize two-photon microscopy for observation of this drug. The two-photon cross section of BPD-MA in a liposomal preparation has recently been measured from 820-980 nm. Having measured the cross section, we will be able to evaluate a detection threshold that will then be ve rified in adherent cell culture. Organelle localization studies have been outlined are expected to proceed in the near future.